diff --git a/client/js/controls/file_dropper_control.js b/client/js/controls/file_dropper_control.js
index f725ee01..a6187209 100644
--- a/client/js/controls/file_dropper_control.js
+++ b/client/js/controls/file_dropper_control.js
@@ -89,7 +89,7 @@ class FileDropperControl extends events.EventTarget {
 
     _evtDragEnter(e) {
         this._dropperNode.classList.add('active');
-        counter++;
+        this._counter++;
     }
 
     _evtDragLeave(e) {